The present invention relates generally to radio frequency (RF) devices, and more particularly to removable electronic circuit modules in which mechanical support, RF shielding the grounding for one or more electronic circuits sensitive to or producing RF signals disposed within are provided.
In the past, electronic instrumentation involving RF circuits have been difficult to repair and service as well as assemble, test and calibrate, because of the interference such operations have on circuit performance. Even where a RF circuit is malfunctioning, repair is difficult because shielding, configuration and calibration of critical components are usually disturbed even while the malfunction is being diagnosed. Thus, such circuit repair often becomes an inconvenient series of trail and error steps involving repeated assembly and disassembly of the circuit components until repaired. Servicing, often merely calibration of instrumentation circuits, may also disturb shielding and configuration of a circuit which, in turn causes an inconvenient iteration of adjustments and component settings.
Typically, RF circuits in electronic instrumentation are isolated by shielding and ground planes from low frequency circuits. However, such RF circuits, which usually include many feedback loops and which often radiate spurious emissions, are often inter-related and therefore not isolated from each other.
RF instrumentation constructed according to the principles of the present invention have all circuits, especially those operating at high frequencies, divided into functional modules. Each module is a section of the instrument taken as a whole, amounting to a subsystem of the instrument, having well defined input and output specifications. Each module has a minimized number of interconnections to other modules of the instrument including, for example, connections for power, digital control and RF signals. Thus, each module contains circuitry which is separately assembled and tested prior to final assembly of the instrument.
Final assembly, test and calibration of a new instrument is greatly facilitated by modularizing according to the present invention. Rapid, on-site field repair of instrumentation such as RF signal generators and analyzers, is now a reality; even if repair of the module itself is not possible, instrument operation can be quickly restored by simply replacing the malfunctioning module. Field servicing is also easy, since separate calibration of each module, none of which interact with other modules while being calibrated, substantially calibrates the whole instrument.
Modules of the present invention include a cast aluminum base having up to two mounting surfaces for mounting electronic circuitry substrates such as printed circuit boards. Each mounting surface may accommodate more than one substrate. The substrates are then each sandwiched between the base and a cast aluminum cover. High frequency gasket material, affixed to the aluminum castings, contact ground plane areas of the printed circuit boards to electrically seal each module. Ground plane areas on the circuit boards extend to the edges thereof and between sections of circuitry requiring shielding from each other thereon. Such ground plane areas incorporate closely spaced, plated-through holes which form a continuous ground plane on both sides of the printed circuit board. All connectors and feed-through filters mounted on the printed circuit boards are grounded to the covers by conductive elastomer grommets.
According to the present invention, the modules are fitted with brackets at each end which slide over corresponding chassis guides mounted on the instrument chassis. Snap fasteners lock each bracket to its respective chassis guide to secure the module in its place in the instrument chassis.
The chassis guides are threaded to receive extension guides which also accept the corresponding snap fastener on each module bracket to lock the module in a servicing position external of the instrument chassis. When the module is in the servicing position, the covers may be removed and the circuits examined while power is applied. The printed circuit boards may also be removed for repair.